From Rigpa Wiki
Vajra (Skt.; Tib. རྡོ་རྗེ་, dorje; Wyl. rdo rje) — a ritual sceptre symbolizing compassion and skilful means, and also a symbol of indestructibility. In tantric rituals the vajra is the necessary counterpart of the bell, which symbolizes the wisdom of emptiness.
Vajra and bell are a set where both have the same number of spokes. There number varies from one to one thousand, yet the most commonly known are the five spoked ones called "samaya vajra and bell" and the nine spoked called "wisdom vajra and bells". The size of the vajra can vary from 4 inches to twenty, and the bell should be in proportion. Since the details of each part are very precise, the omniscient Jikmé Lingpa advised against making one to suit one's own tastes and preferences, but rather to follow the instructions given in the tantras.
Sogyal Rinpoche writes:
- Vajra is compared to the diamond, the strongest and most precious of stones. Just as a diamond can cut through anything but is itself completely indestructible, so the unchanging, non-dual wisdom of the buddhas can never be harmed or destroyed by ignorance, and can cut through all delusion and obscurations. The qualities and activities of the body, speech, and wisdom mind of the buddhas are able to benefit beings with the piercing, unhindered power of the diamond. And like a diamond, the vajra is free of defects; its brilliant strength comes from the realization of the dharmakaya nature of reality, the nature of the Buddha Amitabha.
- Patrul Rinpoche, The Words of My Perfect Teacher (Boston: Shambhala, Revised edition, 1998), page 187.